The present invention relates to thixotropic formulations containing highly disperse silica, to hydroxyfunctional polycarboxylic acid amides of the type described in more detail below, which are used for producing such thixotropic formulations, and to highly disperse silica which has been coated with such hydroxy-functional polycarboxylic acid amides.
A wide variety of thixotropic formulations containing highly disperse silica is known. For generating thixotropy, highly disperse silica is used which is produced by wet or pyrogenic processes (preparation, for example: E. Wagner and H. Brunner, Angew. Chem. 72, 744 (1960)) and normally has a specific surface area (BET method DIN 66131) of about 50 to 400 m.sup.2 /g. Such highly disperse silicas are frequently described in the specialized technical literature also as hydrophilic or colloidal amorphous silicic acids. (See also U.S. Pat. No. 4,238,387)
These silicas are added as a thickening or thixotroping component to liquid systems in diverse fields of application. The typical properties of such silica determine the effectiveness as a thickener and thixotroping agent.
When such silica is dispersed in formulations such as liquids or resin solutions, the chain formations are bonded to each other via hydrogen bonds and form a three-dimensional network. As a result, this process reduces the flowability of the liquid or resin solution. If this network is destroyed by the action of shear forces, the viscosity decreases and builds up again during a certain regeneration time. This process is generally termed thixotropy. Thus, thixotropy is a time-dependent, reversible and repeatable change in consistency, which occurs at the beginning and end of the action of shear forces (J. Schiefer, defazet, Volume 31, No. 2, pp. 62-65 (1977)).
This thixotropy effect is utilized, for example, to prevent settling or running of resin solutions, sealing compounds, adhesives, plastisols, paints, filler compounds and casting resins, and also for imparting a more viscous consistency to, for example, mineral oils. In some formulations, the silica tends to settle. Settling is here to be understood as a non-uniform distribution of the SiO.sub.2 in a vertical plane of the system. This is particularly critical if such silica-containing dispersions are transported for a prolonged period in containers, that is to say are subjected to mechanical stresses. Attempts have been made to counteract this behavior by increasing the SiO.sub.2 concentration in order to thereby reach a point where sufficient SiO.sub.2 chains are present to be linked via hydrogen bonds and thus to build up more shearstable systems. In certain solutions such as, for example, vinyl ester resins in styrene, a thixotropy-providing effect can hardly be obtained by means of hydrophilic SiO.sub.2.
An increase in the SiO.sub.2 concentration up to the stable state is, however, not appropriate in all cases, since excessive SiO.sub.2 concentrations cause faults in many industrial applications, which manifest themselves, for example, in undesired dulling effects in glossy coating formulations or in a decrease of strength in unsaturated polyester resin systems (SPI 21st Meeting: Corrosion resistance polyester resins by P. Trend, D. Edwards and P. V. Bramer).
In formulations which themselves form relatively strong hydrogen bonds, the thixotropy-providing effect of SiO.sub.2 may be substantially weakened. Many manufacturers of highly disperse SiO.sub.2 identify auxiliary agents in their product literature which can increase the thixotropy effect of SiO.sub.2 in various polar systems. Thus, numerous products are described which are said to have a positive effect on the viscosity, thixotropy and the settling behavior of SiO.sub.2 in formulations. These products comprise, for example, low-molecular weight glycols, low-molecular weight diamines, polyamines, carboxylic acids, sulfonic acids, ethylene oxide adducts, sorbitan esters, alkoxylated sorbitan esters, polyethylene glycols and water. (Company brochures: Degussa, Aerosil als Verdickungsmittel fuer Fluessigkeiten [Aerosil as a Thickener for Liquids], and Cabot, "Cabosil" p. 18).
Rotational viscometers are normally used for characterizing the flow behavior. In this case, the shear stress (.tau.) is determined for various shear rates (D). As is known to those skilled in the art, the hysteresis area (L=.tau..multidot.D) between the curve at rising shear rate and the curve at falling shear rate is used as a measure of the thixotropy (company brochure: Haake, Einfuhrung in die praktische Viskosimetrie [Introduction to Practical Viscometry], pages 17-19). These proposed products increase the viscosity of the liquid system without substantially affecting the thixotropy within the meaning of the definition given above, namely the formation of a pronounced hysteresis area (L).
In the case of the low-molecular weight compounds, the positive effects are often only of short duration, since the hydrogen bonds between SiO.sub.2 particles can be broken by rearrangement effects with solvents or low-molecular weight resin constituents. In the case of the amine-containing components, especially in unsaturated polyester resins, vinyl ester resins and epoxy resins, reactions with the resin can occur, so that these compounds are not particularly advantageous for such resin systems. In nitrocellulose coatings and PVC plastisols, amine-containing compounds cause discolorations.
U.S. Pat. No. 4,238,387 discloses that a pseudoplastic rheology of coating compositions can be obtained when these contain finely disperse silica, a polyfunctional acid and an amine in combination. The use of an acid alone or of an amine alone, in addition to the silica, does not produce any effect. It is thus essential that the coating composition contains both the polyfunctional acids or acid anhydrides with carboxyl groups or anhydride groups on the one hand and the amines with their amine groups on the other hand. The amines used can be those with or without hydroxyl groups. Primary, secondary or tertiary amines can be used. The acids used can be certain organic or inorganic acids or anhydrides thereof. At column 2, lines 66 to 69, it is mentioned incidentally that the acid or its anhydride can be reacted with the amine and then added to the SiO.sub.2 dispersion. According to all the teaching in U.S. Pat. No. 4,238,387 this can only be understood as a salt formation. In contrast thereto, it has now been found that the specific hydroxyfunctional polyamides of formula I activate or enhance the effect of silica in various formulations, and particularly even in those formulations in which the mixtures of the aforementioned literature reference are largely ineffective, such as formulations containing vinyl esters as binders.
Among all the auxiliaries described in the literature, none is known which has a wide field of application and, together with SiO.sub.2, produces adequate thixotropy in formulations of different polarity, that is to say which shows wide applicability and produces thixotropy within the meaning of the definition given above, namely the formation of a pronounced hysteresis area (L).